A prior known polarimeter operates in accordance with the principle of automatic optical null balance. The prior polarimeter contains two line radiators, a mercury and a sodium vapor lamp, as well as five optical filters arranged on a filter wheel. A tilting mirror is coupled to the filter wheel shaft. The mirror allows the radiation of the corresponding light source to be inserted into the ray path of the polarimeter at the same time the measuring wavelength is selected. Further, a quartz-iodine and a deuterium lamp are provided to furnish continuous radiation. A grid monochromator is provided to select the desired wavelength.
In this known design, the monochromatic light travels through the polarizer, the cell containing the sample, and the analyzer before arriving at a photo multiplier. The polarizer and analyzer are formed by rotatably arranged "glan" prisms made of calcite. The polarizer, including the respective plane of polarization of the transmitted light, vibrates about the optical longitudinal axis of the system at an excursion of about .+-.0.7 radians at 50 Hz. The 50 Hz signal is generated in the photo multiplier, in the unbalanced state of the system. The 50 Hz signal is amplified and fed, with the correct sign, to the power input of a servo motor. The servo motor is mechanically coupled to the analyzer for rotating the analyzer until the 50 Hz signal becomes zero.
An optically active sample inserted into the ray path causes rotation of the plane of polarization. The analyzer is turned by means of the servo system into the new balanced position. The angular difference between the new and the original balanced position corresponds to the optical capability of rotation of the sample. This type of apparatus, such as the Perkin Elmer model 241 MC, requires mechanical movement of the polarizer, a mechanically acting modulator and a motor rotated analyzer thus making the device unsuitable for miniaturization and/or implantation.
A further prior known method for determining the blood glucose using polarimetry operates by feeding polarized light via a Faraday modulator, operated at a predetermined modulation frequency, through the sample and an analyzer to the detector. In this known method, a further detector is provided because the ray is divided into both measuring and reference signals. Subsequently, the quotient is formed from the two output signals. However, methods using ray division, such as that shown in German Patent DE-OS No. 2944113, require suitable outlays for the optical system. Particularly, the two detectors must meet synchronization requirements.
There is therefore a need for a method and apparatus for measuring the concentration of optically active substances while allowing miniaturization and consequently, implantation into a living body.